Direct Communication has been studied by 3GPP (3rd Generation Partnership Project) (see NPLs 1 and 2).
Key issue for direct communication is to secure an interface PC5. How to secure the interface PC5 and how to establish security context (including for example, key derivation, allocation, update) from trust source, with minimal signaling, are important matters.
Note that the interface PC5 is a reference point between UEs (more than one article of User Equipment) such that the UEs can have direct communication thereover. The interface PC5 is used for control and user plane for ProSe discovery, direct communication and UE relay. The UE direct communication can be carried either directly or via LTE-Uu.